The latest Visual Studio 2019 preview, released yesterday, adds the vertical tabs layout option to the core product:

You can select it in options or right clicking a tab:

In the options you can now additionally select Alphabetical tab sorting (works only for vertical tabs).

There is a scroller when all tabs don’t fit and if the active tab is not visible in the list, its name is displayed in the top right corner:

If you organize several editor windows side by side, tab groups are displayed one below other:

Pinned tabs are displayed on the top and you can freely change tabs area width by dragging the separator:

Alternative tab managers like Tabs Studio and Custom Document Well continue to work as usual if you keep the tab layout option set to the default Top value.

Download Visual Studio Preview to try it for yourself.